Fantasmic! running later this year was kind of a fluke due to Murphy's problems and the Rivers of America being drained in January...but IMO October doesn't exactly match up with what most people think of when they think of "off-season" anymore. It gets very crowded over the weekends and Disney brings out a lot of new stuff to draw people in.
That said, there will be Halloween parties at
Disneyland on Tuesdays and Fridays, I believe it is, meaning the park will close early to guests who don't have a separate ticket. This most likely means at the least:
*No Fantasmic! on Tuesdays and Fridays (since the park will have fewer people due to the Halloween party)
and
*Quite possibly World of Color on Tuesdays and almost certainly on Fridays (since they'll be herding everyone who didn't buy a Halloween party ticket over to DCA those nights).
So they likely can't run Fantasmic! on Fridays in October, and they have a valid nighttime show alternative in World of Color (which I would guess overall costs less to run than Fantasmic!, since there aren't reportedly 80 live actors). So I wonder if they'll just not run Fantasmic! at all in October.
Meanwhile, if they're quite possibly already running WoC on Tuesday, why not the rest of the week? They can easily do 2 shows with a relatively modest (well, modest for Disneyland Park!) 10 PM closing time, or could do one show and close at 9 on weekdays.
I'd be very surprised if they didn't run fireworks on at least weekend (i.e. Saturday-Sunday) nights in Disneyland. My best guess would be it would be the Remember Dreams Come True show, but who knows?